A typical electrophotographic method includes an electrostatic latent image forming process in which an electrostatic latent image is formed on a photoreceptor including a photoconductive substance; a developing process in which the electrostatic latent image is developed into a toner image with a toner; a transfer process in which the toner image is transferred onto a transfer material such as paper; a fixing process in which the toner image is fixed on the transfer material by application of heat, pressure, and/or solvent vapor; and a cleaning process in which residual toner particles remaining on the photoreceptor are removed. The electrostatic latent image forming process further includes a charging process in which a charger uniformly charges a surface of the photoreceptor (e.g., an organic photoconductor (OPC)) prior to formation of an electrostatic latent image.
In particular, the photoreceptor is typically charged by a corona charger. Corona discharge is a continuous discharge phenomenon caused by the occurrence of local insulation breakdown of the air in a nonuniform electric field. A typical corona charger has a configuration such that a wire having a very small diameter is stretched taut within an aluminum shield case, a part of which is eliminated so that corona ions are discharged therefrom. As the wire is supplied with an increasing voltage, a strong electric field is locally formed around the wire and insulation breakdown of the air locally occurs, resulting in corona discharge.
The corona charger generally produces discharge products that degrade the resulting image quality. Specifically, when the photoreceptor is left as it stands after a long-term discharge of the corona charger, the resulting image density may be non-uniform at a portion on the photoreceptor immediately below the corona charger. This is because discharge products are accumulated on inner walls of the corona charger while image formation is occurring and they gradually contaminate the photoreceptor while image formation is not occurring, and as a result, a surface potential difference occurs between a portion immediately below the corona charger and the other portions on the image bearing member.
For the purpose of effectively removing discharge products accumulating on a photoreceptor, one proposed approach involves adding fine abrasive particles to the surface of toner. However, when coarse abrasive particles are excessively present, the photoreceptor may be excessively abraded or scratched and defective images may be produced.
When the content or particle size of the fine abrasive particles is too small, discharge products cannot be sufficiently removed from the photoreceptor or the surface of the photoreceptor cannot be sufficiently refreshed.
Japanese Patent Application Publication No. 2007-156099 proposes fine particles for covering toner surfaces. Such fine particles have an average primary particle diameter of several to several tens nanometer. Hydrophobized silica particles and hydrophobized titanium oxide particles are preferred for the fine particles. As another example, Japanese Patent Application Publication No. 2007-248911 proposes organic fine particles for covering toner surfaces.